High molecular weight (HMW) kininogen (Flaujeac, Williams, Fitzgerald, or contact activation factor) is required in conjunction with plasma prokallikrein for the normal rate of activation of surface-bound unactivated Hageman factor. Turnover studies in rhesus monkeys with 125 I-labelled human and monkey low molecular weight (LMW) and HMW kininogens purified by immunoaffinity and DEAE-cellulose chromatography showed similar half-life values but significantly different fractional catabolic rates. Human plasma prokallikrein has been highly purified in five steps. Isoelectric focusing revealed four major peaks with pI's of 8.5, 8.8, 9.1, and 9.3, besides four minor variants. Human renal prokallikrein (inactive urinary kallikrein) has been partially purified and characterized. Its molecular weight is approximately 50,000 as compared with 35,000 for renal kallikrein (active urinary kallikrein). Trypsin-activated prokallikrein is indistinguishable from renal kallikrein with respect to kininogenase activity. BIBLIOGRAPHIC REFERENCES: Meier, H.L., Scott, C.F., Mandle, R., Webster, M.E., Pierce, J.V., Colman, R.W. and Kaplan, A.P.: Requirements for contact activation of human Hageman factor. Ann. N.Y. Acad. Sci. 283: 93-103, 1977. Meier, H.L., Pierce, J.V., Colman, R.W. and Kaplan, A.P.: Activation and function of human Hageman factor: The role of high molecular weight kininogen and prekallikrein. J. Clin. Invest. 60: 18-31, 1977.